If you think saber toothed tigers and woolly mammoths are cool, just think about what used to live in Australia 45,000 years ago: 1,000-pound kangaroos, 400-pound flightless birds, tortoises the size of Volkswagen Beetles and two-ton wombats.
Scientists have long debated what caused these giant creatures, called megafauna, to go extinct. Some suggested the slow progression of climate change over tens of thousands of years caused their demise as Australia went from being a continent of eucalyptus forests to the dry, arid plains we know today.
But new evidence bolsters the alternative theory that humans, not climate change, were responsible for the extinction of these species. A study published this week in Nature Communications suggests Australia's giant animals went extinct over just a few thousand years—which is fast for well-established species like this.
Researchers said 85 percent of all animals larger than 100 pounds on the continent went extinct shortly after the arrival of humans 45,000 years ago.
"It's a region with some of the earliest evidence of humans on the continent, and where we would expect a lot of animals to have lived," University of Colorado at Boulder Professor Gifford Miller said in a release. They studied soil samples taken from the Indian Ocean off the coast of southwest Australia. "Because of the density of trees and shrubs, it could have been one of their last holdouts some 45,000 years ago. There is no evidence of significant climate change during the time of the megafauna extinction."
Researchers from Monash University in Australia and CU Boulder took soil core samples in Australia going back to 150,000 years ago. They focused in on the spores of the fungus Sporormiella that ate the poop of plant-eating mammals.
The study noted the fungus was abundant in the soil from 150,000 years ago until 45,000 years ago, when they "took a nosedive." This suggests plant-eating mammals started dying out relatively quickly, especially the largest ones that would produce the most poop.
Since there was evidence from ocean soil samples of flourishing forests 45,000 years ago, climate change couldn't have been the cause because the forests hadn't started dying out yet. So humans, and their relentless march across the world, were the most likely cause.
"The results of this study are of significant interest across the archaeological and Earth science communities and to the general public who remain fascinated by the menagerie of now extinct giant animals that roamed the planet—and the cause of their extinction—as our own species began its persistent colonization of Earth," said Sander van der Kaars of Monash University in a release.